Integrated circuit devices are becoming smaller and with fewer signal leads for both cost and space considerations. Typically an integrated circuit device may be in an integrated circuit package having five to eight pins. At least two of these pins must be used for power and ground, e.g., VDD and VSS, respectively. This leaves only three to six pins for useful signal and control purposes. When a plurality of integrated circuit devices, either on a single integrated circuit die or on multiple dice, are in an integrated circuit package, more pins may be used but are still limited due to the number of device functions.
Typical integrated circuit devices whose parameter values are increased and decreased are, for example, digital rheostat, digital potentiometer, digital counter, programmable gain amplifier, modulo N-frequency divider, and digital-to-analog converter (DAC). What is commonly required for these incremental/decremental dependent devices is some way to control incrementing and decrementing the parameter values used by the device. Another desirable feature in these devices is storage in a non-volatile memory of the last parameter value used before power is removed from the device or to protect against a corrupting event such as static discharge or radio frequency interference (RFI). Non-volatile memory may be, for example, one time programmable (OTP) electrically programmable read only memory (EPROM), electrically erasable and programmable read only memory (EEPROM), FLASH memory and the like.
A low pin count device must maximize functionality for each pin of the integrated circuit package. As such, a two (2) wire up/down protocol may use, for example, signal control pins of up/down (U/D) and chip select (CS). However, when using non-volatile memory in such a device, write protection is needed so that the possibility of noise on the U/D and/or CS signals will not cause a desired value stored in the non-volatile memory to be unintentionally incremented or decremented. An unintentional change of a desired value stored in the non-volatile memory could result in a catastrophic application failure. In additional, write protection can insure that factory calibration of a value in the non-volatile memory will remain intact even if the U/D and/or CS signals are not static (intentionally or unintentionally). Generally, a write protection function will require an additional signal pin, e.g., WR or WP pin. Thus the write protection function uses a valuable signal pin of a low pin count integrated circuit package.
What is needed is write protection for an integrated circuit device having incrementing and decrementing parameter values that can be stored in non-volatile memory such that only two signal nodes or pins are required for all of these functions, including write protection of the non-volatile memory.